Various silver halides which form high contrast images are well known, and photographic image formation methods employing such silver halides are used in the field of photoengraving.
Special developing solutions known as lithographic developers were used in the prior art for this purpose. Lithographic developers contain hydroquinone as the developing agent; and sulfite as a preservative which does not inhibit infectious development. The sulfite is used in the form of a formaldehyde adduct, and the concentration of the released sulfite ion is very low (generally 0.1 mol/liter). As a result, lithographic developers are easily oxidized by atmospheric oxygen, and disadvantageously cannot be stored for more than 3 days.
Methods using hydrazine for obtaining high contrast photographic characteristics using stable developers are disclosed in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,211,857, and 4,243,739. Supercontrast high sensitivity photographic characteristics are obtained by the practice of these methods; furthermore, because these developers can tolerate a high concentration of sulfite, their stability against air oxidation is greatly increased in comparison with the lithographic developers.
Nevertheless, the above noted methods of forming supercontrast images using hydrazine compounds are characterized in that the infectious development progresses strongly. As a result, when photographing a low contrast character original (particularly the fine lines of Ming style type), portions which should have been fine white regions become blackened, the characters also become blackened and collapsed and cannot be deciphered. Because of this, if the exposure is reduced to provide better reproduction of the fine lines of the Ming type characters, Gothic type letters become blackened. Namely, the problem was that the latitude of exposure was narrow. A similar problem arises in photography of dot images, namely, blackening easily occurs up to the portion which is the white ground of the dot, and the dot gradation has the disadvantage of very defective image quality.
Due to the strongly infectious development characteristics of a hydrazine compound, the portions with low exposure or which are unexposed and are located adjacent to the exposed portions are as a result also developed. In order to inhibit this undesirable effect, a method of restraining image broadening due to infectious development, and a method for development (below micro development inhibition) to restrain the development of portions adjacent to the image portion, was desirable.
As a method of inhibiting infectious development, the quantity of nucleating agent may be reduced, and the pH of the developer may be lowered as disclosed in JP-A-1-179939 (corresponding to U.S. Ser. No. 295,671( (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"); however, the gradation becomes soft toned, and the degree of sharpness of image lines is lost.
The present invention controls the nucleating agent by adjusting the pH of the developer to 11.2 or less. A sufficient increase in contrast generally does not arise when the pH is less than 11.2, but sufficient gradation contrast may be obtained by the conjoint use of a nucleation promoter. In development at this pH, in comparison with development at a higher pH, the present inventors have discovered that the tendency to infectious development is weak, and image broadening is limited.
Furthermore, the development reaction in the image portion is invariably accompanied by the release of H.sup.+ ions and halogen ions. The pH is reduced by the diffusion of H.sup.+ ions to portions adjacent to the image, and micro development restraint of the portion adjacent to the image is caused by the diffusion of halogen ions, but it is recognized that this development is readily manifested in nucleation development at a pH below 11.2.
Image reproducibility is greatly increased by these various methods, as shown by JP-A-63-257658, but further improvement is desirable. Photosensitive materials containing redox compounds which, upon oxidation, release photographically useful groups are disclosed in JP-A-61-213847 and U.S. Pat. No. 4,684,604, and proposals for widening the processing window for obtaining good gradation are disclosed. In supercontrast processing systems in which hydrazine derivatives are used, these redox compounds have the disadvantageous effect of inhibiting greater contrast, such that optimum results are not obtained.